The rules of typical market saturation do not apply to mobile phones. Customers tend to upgrade every 18-24 months and start new contracts.
So 2 million customers sign up today, you effectively get the same 2 million customers again in 24 months. New phones/New deals means customers are always swapping networks. Sky will have a mobile network, you're just going to have accept it.
We were once told they would never match VM for BB speeds, they now can. We were told they will never match VM for VOD, they have. An they WILL

have a mobile network.